Internal combustion engines are provided with a forced-circulation lubrication circuit which maintains a lubricating oil film between all mechanical parts undergoing relative movement, in order to reduce friction between the parts and to remove and dissipate the heat generated.
With the passage of time, impurities accumulate in the lubricating oil. These impurities consist mainly of metal particles deriving from the wear of the engine sliding members, carbon particles produced by the fuel and by the combustion of seeping oil, and mineral powders, generally siliceous and calcareous, derived from the dust suspended in the engine intake air.
In order to eliminate such solid particles, which with the passage of time lead to considerable wear of the moving parts of the engine, a single filter cartridge is usually provided. The cartridge is generally in the form of a thin sheet metal casing containing a folded porous paper cylinder, together with a valve system for controlling the passage of oil.
These filters also comprise a pressure relief or bypass valve. The pressure relief or bypass valve assures oil feed to the engine even when the oil pressure downstream of the valve is insufficient, due either to operation of the engine at high speed or to clogging of the filter.
When the pressure relief or bypass valve opens it ensures correct lubrication, even if the filter is clogged. On the other hand, the valve also enables not only the oil but also the impurities contained in the oil to circulate through the engine members, to produce the damage mentioned previously.
It is therefore particularly important to determine the state of clogging of the oil filter in order to be able to replace it as soon as it has exceeded its designed operating state and before the pressure relief valve opens or before the oil passing through it undergoes a high pressure drop, without having to rely on an estimate of average life as in the case of filters known up to the present time.
The filter element currently used is generally not able to retain solid particles of less than 20 microns in size. Higher filtration power would lead to an excessive pressure drop in the oil as it passes through the filtration surface. It is, however, apparent that a filtration system able to retain solid particles of just a few microns in size would considerably reduce the wear of the moving engine parts.
Up to the present time the filtration methods used include the provision of a second filter element of greater filtration power than a first filter but traversed by only a certain proportion of the total oil flowing through the lubrication system. Thus, after a certain time, all the fluid will have passed through the second filter element without creating excessive pressure drop in the circuit.
The main problem with such a filtration method is that the second filtration surface, offering considerable flow resistance, is in parallel with a flow path offering negligible resistance.
It is therefore necessary to provide a pressure drop between the upstream and downstream sides of the second filtration surface to overcome the flow resistance provided by the filtration surface. This has been achieved up to the present time by dividing the outlet flow from a conventional filter into two paths, one directed to the engine and the other directed to a second filter of greater filtration power. The outlet pipe of the second filter returns the outlet flow to the oil sump so that the pressure drop between the pressurized circuit upstream of the second filter and the oil sump downstream of the second filter overcomes the resistance to flow through the filtration surface of the second filter.
This method is disadvantageous in that it leads to considerable complication of the lubrication circuit.
Also known are double filters having one inlet and two outlets. One outlet is directed to the engine and the other to the sump. The filter according to the present invention offers the advantages of this type of double-path filter without requiring modifications to the vehicle engine. These advantages are attained by means of a secondary path formed by a Venturi tube.